In the prior art, there has been considerable interest in developing methods and apparatus for evaluating the composition and thickness of thin films on a substrate. This need is particularly acute in the semiconductor manufacturing industry where extremely thin films are deposited on a silicon substrate.
The preferred devices rely on non-contact, optical measurement techniques. In these devices, a probe beam is directed to the sample, and a particular parameter of the reflected probe beam is measured. For example, it is known that as the thickness of the film varies, the intensity of the reflected probe beam will vary due to the variation in interference effects created at the interface between the thin film and the substrate. It is also known that the thickness of the thin film will have an effect on the change in polarization state which occurs when the probe beam is reflected off the sample surface. Thus, by monitoring either the change in intensity of the reflected probe beam or its change in polarization state (ellipsometry), information about the thin film can be derived.
In most devices, some form of multiple measurements are desirable in order to increase accuracy. One such approach for gaining additional accuracy is to obtain measurements at a number of different wavelengths. A spectrophotometer is designed to provide interferometric type measurements at various wavelengths. Another approach for gaining additional accuracy is to take measurements at a number of different angles of incidence of the probe beam.
Until quite recently, the mechanisms for generating measurements at multiple angles of incidence were quite cumbersome. More specifically, the equipment had to be designed so that the angle between the probe beam optics and the sample could be varied.
It has been found that these difficulties can be overcome by extracting angular information from rays within the reflected probe beam. This approach is described in detail in U.S. Pat. Nos. 4,999,014 and 5,042,951, assigned to the same assignee herein and incorporated by reference. As described in these patents, the light from the probe beam is tightly focused on the sample surface in a manner to create a spread of angles of incidence for individual rays within the focused probe beam. After reflection, individual rays within the probe beam are analyzed, with the radial position of the rays within the probe beam being related to the angle of incidence of the beam on the sample surface. Preferably, a photodetector having an array of individual elements is used to measure rays having different angles of incidence. This approach can be used in both interferometric and ellipsometric analyses.
In the preferred embodiments discussed in the above identified patents, the probe beam was generated by a laser having an output that was substantially diffraction limited allowing focusing to a spot size on the order of one micron in diameter. This approach provides high spatial resolution permitting analysis of extremely small regions on the sample.
In many cases, such high spatial resolution is unnecessary. In fact, in some analyses, the semiconductor manufacturer is only interested in measurements of average conditions over a much larger region. In the latter case, it would be possible to use a non-diffraction limited light source such as an arc lamp or other white light source.
One advantage which is obtained when utilizing a white light source is that additional measurements can be taken at different wavelengths, in a manner analogous to commercially available spectrophotometers. In these devices, a means is provided for sequentially or simultaneously selecting a plurality of individual wavelengths. The means can include various combinations of filters, gratings or prisms.
As can be appreciated, the need to provide wavelength selective elements adds to the cost and complexity of the apparatus as well as requiring moving parts. More significantly, the need to take sequential measurements slows the operation of the device. Therefore, it would be desirable to be able to obtain simultaneous measurements not only at different angles, but at different wavelengths as well.
One approach that was suggested for achieving this goal is set forth in U.S. Pat. No. 5,042,951, to Spanier, which is incorporated herein by reference. The device disclosed therein is a simultaneous multiple angle of incidence ellipsometer. One embodiment of the device includes a polychromatic light source. FIG. 5 of the patent illustrates a method by which it is suggested that simultaneous measurements can be made at both multiple angles of incidence and at multiple wavelengths. This approach includes providing a dispersing element for spreading out the beam as a function of wavelength. The beam is then directed to a photodetector having a two dimensional array of individual detector elements. The array is oriented so that each column measures light from only a narrow band of wavelengths at a plurality of angles of incidence. In contrast, each row is arranged to obtain measurements at a single angle of incidence at various wavelengths.
The approach described in the Spanier patent was suggested because of the desire to generate simultaneous measurements at various angles of incidence and at various wavelengths without scanning either variable. However, the results that can be achieved using the approach described in Spanier are less than ideal. The problems associated with the Spanier proposal can best be understood by referring to FIG. 1 herein. FIG. 1 illustrates a two dimensional photodetector array 10 of the type proposed by Spanier. The array includes a plurality of rows 6 and columns 8 of detector elements. Superimposed on top of the array are the foot prints of two different probe beams (2 and 4) having different wavelengths. The pattern of the probe beams (2, 4) would be of the type created using a dispersing element to spatially separate the wavelengths in a polychromatic beam. In this illustration, the dispersing element is oriented in a manner to separate light of different wavelengths along a vertical axis so that the detector elements across each row 6 measure light as a function of angle of incidence.
Arrow A in FIG. 1 is aligned with the row of photodetector elements associated with the central diameter of beam 2. As taught in the above cited patents of the assignee, the angle of incidence measured by any individual detector element in the row is based on the its radial position with respect to the beam. Specifically, the radial position (R) is proportional to sin.theta./M where .theta. is the angle of incidence and M is equal to the maximum angle and is given by the following equation: EQU R=R.sub.M (sin.theta./sin.theta..sub.M) (1)
Based on these relationships, it can be seen that the center detector element 12 will correspond to the central angle of incidence of the beam. The left and right-hand detector elements 14 and 16 will correspond to the maximum and minimum angles of incidence in the beam. In the assignees preferred design, wherein the beam is focused normal to the surface of the sample with a high numerical aperture lens (i.e., 0.90NA), the spread of angles of incidence is on the order of 128 degrees. In this case, the detector elements 14 and 16 at the radially outermost points on the beam will measure rays having angles of incidence of +64 degrees and -64 degrees.
The problems of the Spanier design can best be appreciated by considering the light falling on an intermediate element 18 where the two beams overlap. For beam 2, element 18 lies halfway between the center and the edge of the radius of the beam. In a system having a probe beam at normal incidence and a half angle spread of 64 degrees, the angle of incidence of a ray falling halfway between the center and the edge of a beam would be on the order of 27 degrees.
As can be seen in FIG. 1, the light falling on element 18 is not limited to light associated with the wavelength of beam 2. Rather, light from beam 4 of an adjacent wavelength falls on element 18 as well. Having light of different wavelengths fall on individual detector elements will complicate the wavelength dependent analysis of the sample. More significantly, the light falling on element 18 from beam 4 corresponds to a substantially different angle of incidence than the light from beam 2. This difference can be appreciated by comparing the radial distance between the center of beam 2 and element 18 (R1) with the radial distance between the center of beam 4 and element 18 (R2). As can be seen, the radial distance R2 is significantly greater than the radial distance R1. Recalling that the angle of incidence is directly related to the radial position of the rays within the beam, it can be seen that the angle of incidence for the light ray of the wavelength of beam 4 at element 18 is larger than the angle of incidence for the ray associated with beam 2 at element 18. In the illustrated example, the angle of incidence of the ray for beam 4 at element 18 would be on the order of 46 degrees as compared to the 27 degrees for beam 2.
The example illustrated in FIG. 1 is simplified in that only two wavelength beams are shown. In reality, there will be multiple overlapping beams. Thus, the approach described in Spanier results in each detector element detecting light not only from multiple wavelengths but many different angles of incidence as well. This overlap results in a substantial blurring of the data preventing an adequate analysis of the sample. Accordingly, it would be desirable to provide a detection approach which provides significant isolation for the wavelength and angle of incidence measurements.
Therefore, it is one object of the subject invention to provide an improved method for simultaneously measuring both angle and wavelength information contained in a reflected probe beam.
It is a further object of the subject invention to provide a method of simultaneously measuring angle and wavelength information which can be used in both interferometric and ellipsometric devices.
It is still a further object of the subject invention to provide a simultaneous multiple angle of incidence interferometric device which generates measurements at a plurality of selected wavelengths.
Another aspect of the subject invention relates to an approach for easily varying the area on the sample over which measurements are taken. As noted above, when using a laser to generate the probe beam, a spot size on the order of one micron can be obtained using a fast lens. When using a white light source, such tight focusing is not possible and a much wider area is illuminated. When using a high numerical aperture lens in a configuration as described in the assignees two prior patents, the minimum focused spot size which could be achieved with a white light source would be on the order of 150 microns in diameter.
Accordingly, it would be desireable to provide a system wherein the selected measurement region can be less than the minimum focused spot size diameter. Moreover, it would be desirable to provide a device wherein the size of the measurement region can be easily changed without moving or replacing the lens elements.